Applicators are routinely employed in many diverse industrial applications to apply a pattern of a material, such as one or more beads of an adhesive, to each of a series of workpieces being sequentially transported on a conveyer past the applicator. In automated packaging production lines, for example, adhesive applicators apply one or more amounts or beads of hot melt thermoplastic adhesive to joint flaps of blanks that are subsequently folded to assemble adhesively-bonded boxes, cartons, or other containers. Hot melt thermoplastic adhesives are commonly used in such packaging applications where the rapid setting time of this type of adhesive is beneficial.
Assembled containers are eventually filled with an amount of a product and sealed to form a closed vessel with the product confined inside the closed vessel. If the applied adhesive is improperly positioned, gaps may be present between the joint flaps or the joint flaps may separate or be partially breached, for example, during shipping. This lack or absence of seal continuity causes a loss of product confinement and may result in leakage of, or damage to, all or part of the held product. Therefore, it is desirable to detect improper placement of the adhesive bead(s) after the adhesive is applied and without individual inspection of the containers.
Applicator systems on high-speed variable velocity production lines require that the response time for the applicator be adjusted to apply the adhesive at the desired location(s). Because of intrinsic mechanical and electrical system delays, such applicator systems require response time compensation to accurately place the adhesive on the workpiece. The response time compensation corrects for time delays between the instant that an electrical pulse is sent by a controller to the applicator and actual adhesive contact with the workpiece, and similar delays in discontinuing adhesive application. Contributing factors include the time-of-flight of the airborne adhesive in traveling from the applicator to the workpiece, transducer delays, delays arising from inductance of solenoid coils in solenoid-operated applicator valves, and delays due to the mechanical response time of the applicator valve.
In one conventional approach for setting response time compensation, a production line operator empirically measures the location of the adhesive and manually enters a response time compensation during a system initialization or start-up phase. This procedure has substantial potential for error, as the operator may incorrectly measure the location, or may incorrectly program the controller. Once the response time compensation is set, changes in operating parameters (i.e., changes in adhesive pressure, adhesive viscosity, or line velocity) may cause unwanted shifts in adhesive placement on the workpiece. In an iterative procedure, the operator must measure the applied location of the treatment and adjust the response time compensation until the applied location matches the desired location. This iterative procedure is a time consuming process as it requires several repetitions, thereby reducing line productivity.
Conventional automated adhesive applicators for high speed variable velocity production lines regulate adhesive placement by monitoring system operating parameters. Such automated applicator systems have an encoder that senses the line velocity of the conveyor, an applicator, an adhesive sensor that monitors adhesive placement on the workpiece, and a position detector that senses the presence of a portion (i.e., leading or trailing edge) of a conveyed workpiece at a known distance from the applicator nozzle. The controller of a control unit orchestrates the operation of the applicator (i.e. opening and closing of the applicator's solenoid-operated valve) in response to the signals received from the encoder, the adhesive sensor and the position detector.
The system control unit opens the applicator valve to discharge adhesive, which is supplied under pressure to the applicator, in a predetermined pattern through the applicator's nozzle. The system control unit also closes the valve to halt the discharge of adhesive from the nozzle. The discharge of adhesive is synchronized with the line velocity to achieve proper adhesive placement on the workpiece. The duration over which the valve is open, in conjunction with the line velocity, defines the length of the dispensed adhesive pattern. Signals supplied from the encoder and position detector to the system control unit determine the timing of trigger signals that open and close the applicator. The adhesive sensor detects the actual location of the applied adhesive.
An operator selects initial values for response time compensation from charts, or other references, during the system start-up phase and enters these initial values into a system controller for the automated adhesive applicator. An iterative procedure is used to adjust the response time compensation to provide accurate adhesive placement. The response time compensation is presumed to change linearly with a change in the line velocity. However, this predictive approach neglects any changes in the line velocity (i.e., acceleration) that may occur. The predictive approach also ignores any changes in other operational parameters, such as adhesive pressure and temperature.
It would therefore be desirable to provide an improved control system and method for regulating the placement of adhesive on conveyed workpieces in high-speed variable velocity production lines that can more accurately account for changes in operating parameters such as, for example, adhesive pressure, adhesive temperature, and line velocity.